The use of a hollow or solid rectangular (or square) reflective light tunnel or solid glass rod (herein collective referred to as "light tunnels") is well known as a non-imaging device that can generate a "uniform" intensity distribution from a non-uniform spatial intensity distribution produced by a light source, such as from an arc lamp or laser.
Most light tunnels are designed to provide uniformity of illumination up to about the 95% level (i.e., 5% non-uniformity). However, for certain applications such as photolithography and laser thermal processing (LTP), the spatial uniformity needs to exceed 99% (i.e., less than 1% non-uniformity). To achieve this level of uniformization, the light tunnel must have a length to width ratio on the order of 100:1 to provide the necessary large number of homogenizing reflections (typically more than 4 reflections in each direction).
FIG. 1 shows an example of a non-imaging light tunnel 10 having an optical axis A along the z-direction, a width W=5.7 mm and a length L=500 mm. Light tunnel 10 is solid and may be made of optical glass, such as fused silica having a refractive index n=1.45 in the visible spectrum. With reference also to FIG. 2, light tunnel 10 is polished with six mutually perpendicular surfaces S1-S6, which includes 4sides (S1-S4), an input end (S5) and an output end (S6). Surfaces S1-S6 meet at sharp edges and are straight and flat over their extent. Light tunnel 10 having such dimensions is very fragile and difficult to fabricate. FIG. 3 shows light tunnel 10 having the proportion W/L of 1/100 for the sake of illustration. The above dimensions are close to a practical limit of both manufacturability and sensible cost.
In use, light tunnel 10 functions as follows. With reference again to FIG. 1, a light beam 20 comprising light rays R (including a straight-through light ray R.sub.T) from a light source 26 is relayed via an optical relay system 28 and converged onto input end S5 through an input angle .alpha.. The spatial extent of light beam 20 imaged onto input end S5 is typically somewhat circular with a reasonably symmetric spatial intensity distribution. The spatial uniformity of light beam 20 in the x and y directions is shown schematically in FIG. 4A. Light beam 20 is preferably defocused so that it just underfills input end S5 and does not concentrate energy above the Laser Damage Threshold (LDT) of the optical glass from which the light tunnel is fabricated.
A reasonable value of a is found to be about .+-.10.degree. (half-cone angle), which corresponds to a numerical aperture (in air) of 0.18 or an f/# of f/2.8. Larger numerical apertures present additional problems in managing the output illumination, which has an output angle .alpha., the same as the input angle. Somewhat "faster" f/#s can be used, depending on the complexity and expense the optical system can endure for the downstream relay optics 30 that relays light from output end S6 through other sections of the optical system (not shown).
It is known in the art that the longer the light tunnel, the greater the spatial uniformity of the illumination formed on output end S6 because the spatial uniformity increases with a larger number of reflections, and, for a given input f/#, it is possible to have a greater number of reflections with a longer light tunnel. Normally, output end S6 has cross-sectional dimensions W.sub.x and W.sub.y, so that the light tunnel may be square or rectangular. In FIG. 1, W.sub.x =W.sub.y =W, for the sake of simplicity.
With continuing reference to FIG. 1, all light rays R in beam 20 entering input end S5 of light tunnel 10 exit from output end S6, provided that the refractive index of the optical glass n&gt;(2).sup.1/2 or 1.414 . . . for the given wavelength of light used. Virtually all optical glasses exceed this value. Hence any light ray that enters input end S5 at .alpha.=90.degree. or less will refract and be guided down the tunnel's length by multiple-reflections from surfaces S1-S4 of light tunnel 10, and exit at .+-..alpha. from output end S6. Every light ray R will undergo an "even" or "odd" number of reflections, depending on the length L and incident input angle .alpha.. Because light rays R in a solid light tunnel 10 undergo Total Internal Reflection (TIR), there is no light loss internally due to reflection. Absorption and scattering in the optical glass and "end" losses due to Fresnel surface reflections are the only losses encountered. Anti-reflection coatings can minimize the latter.
It is possible to construct a rectangular hollow light tunnel 10 by butting together four mirrors. Other than the "internal medium" being air with a refractive index of 1.0, the geometrical behavior is, to first order, identical to that of a solid light tunnel. For a given width W, a hollow light tunnel 10 will be the shortest embodiment for a given number of reflections or "bounces" of rays R from input end S5 to output end S6. That is, a hollow light tunnel 10 will produce the most uniform output distribution in the shortest length L. Even so, the length L must be great, requiring long slender mirrors. The edges that butt to the surface of the adjacent mirror must be sharp. Dirt on the inside reflective surfaces can be a practical problem. The inner surfaces of the mirrors preferably include an optical coating designed for grazing incidence reflection to avoid excessive polarization and selective absorption. A mirror-based hollow light tunnel will not be as efficient as a solid glass light tunnel when reflection losses from the mirrors are compared to TIR of solid glass.
For given values of .alpha., W.sub.x, W.sub.y, L and n, the number of reflections that occur on either side of the directly transmitted ray R.sub.T traveling along axis A is limited by .alpha. and will be equal to N.sub.xy, which is given by: EQU N.sub.xy =.+-.Tan(Sin.sup.-1 (1/(2.times.n.times.f/#))) (L/W), (Eq-1) EQU where f/#=1/(2 Sin .alpha.)=1/(2NA) (Eq-2)
For a circular beam, the total number of reflections is given by N.sub.tot : EQU N.sub.tot =(.pi./4)(2N.sub.xy +1).sup.2 (Eq-3)
For L=500, W.sub.x =W.sub.y =W=5.7, n=1.43, and f-number=f/2.8, N.sub.xy =.+-.11 reflections either side of the transmitted beam, which gives EQU N.sub.tot =415 reflections.
The angular subtense of a single beam is given by: EQU .DELTA..alpha.=Tan.sup.-1 (nW/L) (Eq-4)
which for .alpha.=.+-.10.30 half-cone angle results in each beamlet subtending an angle of 0.95.degree..
For small angles .alpha., the number of reflections N.sub.xy, is approximated by: EQU N.sub.xy.congruent.(.+-..alpha.)/(nW/L) (Eq-5)
Hence, there will be at output end S6 an average of 415 superimposed images of input end S5. The result is a highly spatially uniform distribution of light I.sub.out at output end 56 that is sharp-edged and behaves as an ideal light source. This is illustrated in FIG. 4B.
The problem with the foregoing is that when W needs to be much larger than 5 mm, or L much longer than 500 mm (i.e., about 20 inches), serious fabrication difficulties are encountered. In particular, the light tunnel is slender and fragile and thus difficult to handle and easily damaged in processing (see FIG. 3). It becomes necessary to increase the width, W, of the tunnel when high intensity light sources, such as lasers, are used. For example, lasers with high energy/pulse characteristics (approximately greater than 5 joule/cm.sup.2 /pulse with a wavelength in the visible range on most conventional glass tunnels) may exceed the damage threshold. However, increasing the width of the light tunnel necessarily requires that the length also increase so as to maintain the number of reflections required to achieve adequate uniformity. Very quickly, the size of the light tunnel becomes unmanageable.
To reduce the unwieldiness of long light tunnels, it would be desirable to fold them in a manner that allows for the redirection of light with little or no light loses, or increase in etendue. The physical significance of "etendue" or equivalent terms (Lagrange Invariant, optical invariant, etc.) can be stated as follows: the product of the solid angle irradiated by an emitting source and its radiant power-weighted surface area is "constant" when "imaged" throughout an optical system. In other words, EQU (Power).times.(Source Area).times.(Source Solid Angle)=Constant
Simply put, if an optical system is 100% efficient, it can only re-image the source at another location with an equivalent "luminance." Since, owing to losses in optical systems (e.g., surface reflections, vignetting, material absorption and scattering), optics efficiency will be less than 100% and an image of the source will always have a lesser "luminance" than the source itself.
Also important however is the matter of spatial uniformity of an image of the source. A "uniformized" image plane has a uniform illumination distribution. A light tunnel comprises multiple sources (reflected beamlets of the main source), all of which contribute to each point within the illumination distribution at the output end. It can then be said that this surface will have a spatially uniform "luminance" regardless of the "etendue." Hence, any subsequent image of the surface, regardless of whether it is magnified, will produce an image that is as spatially uniform, provided that optics used to re-image this surface are suitably designed.
U.S. Pat. No. 5,852,693 ("the '693 patent) discloses a light-tunnel device that includes the use of "special redirection members" and "special coupling members" to fold the light tunnel. The "special coupling members" are used to join shorter prismatic sections of rectangular or square light tunnels together to form the equivalent of a long light tunnel. However, a significant disadvantage of the '693 patent is the necessity for coupling members in the form of prisms that connect square-ended light tunnel segments ("rods") together. The coupling prisms need to have a higher-refractive index than the "rods." This arrangement is designed to reduce the divergence of rays leaving each rod through a coupling prism and thereon entering the next rod, etc. In the '693 patent, the joints between adjacent prisms and rods need to have some special treatment, e.g., be air spaced when total internal reflection is useful or desired. Alternatively, the joints need to be optically bonded with a suitable adhesive when total internal reflection is not required. Further, the joints need to be coated with a thin-film of a appropriate dielectric layers and/or metal to enhance reflection from either a rod to prism interface or at the angled prism face that would ordinarily be left uncoated for TIR.